The invention relates to an apparatus for measuring properties of a moving paper web or cardboard web the apparatus comprising a sensor and a counterpart, the sensor being arranged on a first side of the web and the counterpart on a second side of the web, the sensor comprising at least one coil.
Measurements of properties of a moving paper web constitute the basis for controlling a paper machine. Continuous monitoring of properties: e.g. the caliper of paper, as the web moves, ensures that paper of the right quality is produced. The problem in measuring the caliper of a moving web is that the web moves at an extremely high speed and that the web always comprises some discontinuities, such as holes and small bulges. Measuring sensors located very near the surface of the web, or even in contact with it, should be able to give way to the discontinuities in order for the measurement to be reliable and the sensors not to cause additional hole formation. Present devices are not capable of giving way sufficiently rapidly, and consequently formation of holes has been prevented e.g. by keeping sensors far away from the web. This impairs measuring results.
U.S. Pat. No. 4,791,367 discloses a gauge for measuring the caliper of a web. The gauge comprises an upper head having a U-shaped magnetic pole piece whose legs are provided with windings. A lower head to be arranged on the opposite side of the paper web to be measured comprises a passive magnetic circuit which is a magnetic or ferrite plate. The magnetic flux established by the windings extends through the pole piece through the paper to the ferrite plate. The gap between the pole piece and the ferrite plate varies as the caliper of the paper web varies. The circuit inductance used to define the caliper of the paper web also varies simultaneously.
U.S. Pat. No. 4,528,507 discloses a solution for measuring properties of a moving web by sensors arranged on different sides of the web. One of the sensors is provided with a coil, the sensor on the opposite side of the web being provided with a counterplate. As the gap between the coil and the counterplate varies, the density of the magnetic flux established by the coil varies and the caliper of the web is measured by detecting the variation in the density.
German Offenlegungsschrift 28 29 264 discloses a measuring apparatus, in which a sensor including a coil is arranged on both sides of the web. The caliper of the web is measured by defining the amplitude of the magnetic field established by the coils.
In all the above solutions, the sensor coils, typically made from copper wire, are heavy, making the sensors heavy and difficult to move flexibly along formations on the surface of paper. Furthermore, because the construction is heavy, the sensors are also very poorly adapted to float by air bearings, completely clear of the surface of paper.
It is an object of the present invention to provide an apparatus for avoiding the above drawbacks.
The apparatus of the invention is characterized in that the coil comprises a plurality of layers, each layer constituting a part of the coil, the layers being superimposed and coupled in series to constitute the coil.
It is an essential idea of the invention that the apparatus for measuring properties of a paper web comprises at least one coil comprising a plurality of coil parts arranged as superimposed layers and coupled in series at their ends to form an integral coil. It is the idea of a preferred embodiment that the injection frequency used in the solution exceeds 50 kHz.
It is an advantage of the invention that the sensors of the measuring apparatus of the invention can be made lightweight, whereby the sensors are able to give way to irregularities on the surface of the paper more flexibly than before. This decreases formation of holes in the paper. The lightweight construction of the invention also allows the sensors to be made such that they float by means of air bearing. The construction can be made lightweight, since, owing to the high injection frequency, the measuring apparatus requires no heavy ferromagnetic materials, such as iron, ferrite or a large conductor coil made from copper wire. In the solution of the invention the number of wire turns of the coils can be low, resulting in a small and lightweight measuring apparatus.